1. Field of the Invention
This invention relates to a method of making a magnetic oxide film for a magnetic disk or the like, and more particularly to a method of making a homogeneous magnetic medium of high coercive force with high efficiency and with ease.
2. Description of the Prior Art
For enhancement of the magnetic recording density of a file memory, in particular, a magnetic disk, the following two points are generally indispensable: to increase the coercive force of a recording medium and to form the medium as thin as possible.
Conventional methods for the manufacture of a magnetic oxide film for magnetic disks include a particulate coating method, a method of forming a film of Fe.sub.2 O.sub.3 from a solution of ferric chloride and reducing it to magnetic oxide and a method of forming alternating layers of magnetic iron and its oxide upon a substrate by controlling a sputtering atmosphere.
The particulate coating method employs a binder to provide for enhanced adhesiveness of fine oxide particles to a substrate and increased tightness of bonding of the particles. This reduces the density of the magnetic oxide contained in the resulting film, and hence is not suitable for obtaining high magnetic recording density. With the present coating techniques, it is very difficult to reduce the thickness of the film to less than 0.3.mu.m. Further, it is also impossible to increase the coercive force of the film more than 500 Oe.
With the method of forming a film of Fe.sub.2 O.sub.3 from a solution of ferric chloride, and reducing it to magnetite (U.S. Pat. No. 3,620,841 (1971)), since the thickness of the film coated at one time is small, it is necessary to repeat the process of coating and drying the film a plurality of times, and consequently this method is low in work efficiency.
Further, the method of forming alternating layers if magnetic iron and its oxide upon a substrate by controlling a sputtering atmosphere (U.S. Pat. No. 3,829,372 (1974)) requires apparatus for controlling the oxygen partial pressure in the sputtering process and involves a control of the absolute values of the thicknesses of iron and iron oxide to provide for increased coercive force of the thin film and maintain it at the constant value. This method has the advantages of requiring sophisticated apparatus and a high degree of control.